The present invention relates to a heating element in which a powder of a metal such as iron, zinc, etc. is oxidized with oxygen contained in air, and utilizes heat energy (heat of oxidation) accompanying the oxidation process.
Known heating elements used in heater poultices or disposable pocket heaters utilize heat of oxidation of a metal powder. Processes for the oxidation of a metal powder may be classified roughly into two categories, one a process in which metal oxides or chlorides having an ionization tendency lower than that of metals to be oxidized are used, and the other a process in which oxygen is adsorbed on a non-metal such as active carbon to utilize the oxidation action thereof. However, the former process in which metal oxides or chlorides are used has problems that they have only a short life, the metal oxides used as an oxidizing agent are dangerous and the metal chlorides have toxicity, although a high temperature may be attained (a temperature above 100.degree. C. is possible) and a high calorific value per unit time can be obtained. On the other hand, the latter process in which oxygen in air is adsorbed on active carbon, silica gel, activated alumina or zeolite containing water (in some cases a salt such as sodium chloride is added) to impart an oxidizing capability to the latter has demerits in that the temperature of heat generated is generally low (average temperature being about 60.degree. C. under even the optimum conditions), reaction velocity is low, and calorific value per unit time is low, resulting in a lower effect of warming the body when the heater according to the process is used outdoors in bitter cold winter days. This is because, even if it provides temperatures of 60.degree.-70.degree. C., the temperature will be lowered to 45.degree.-50.degree. C. when the heater is applied to the body, despite its long life and high safety. Packing of the heating element using oxygen in air is effected such that active carbon of a particle size of usually 80-100 mesh impregnated with an aqueous solution of a 10-20% concentrated salt such as sodium chloride, magnesium chloride or calcium chloride is placed on one side of an air-permeable bag, a powder of a metal such as iron or zinc is placed on the other side of the same bag, and the bag is partitioned at the center with, for example, a clip to prevent both powders from being mixed. The whole bag is enclosed airtight in an air-impermeable outer bag. For use, the inner air-permeable bag is taken out from the outer air-impermeable bag, and both powders are then mixed together to generate a heat. Alternatively, a mixture of both powders is previously mixed in an air-permeable bag, but it is isolated from air. For use, it is exposed to air. However, in the case of the former package containing the two different metal powders separately from each other, the metal powders are solidified into granules due to humidity during storage or non-use. Consequently, a sufficient heat generation cannot be obtained in an actual use. In the latter package containing previously mixed metal powder and active carbon powder, the powder coagulation may be less, but it is also caused if nonuse continues for three months or longer. Furthermore, in using active carbon, water penetrates into the pores thereof during storage or non-use to cause a wetting phenomenon, thereby reducing its oxygen-adsorption capability. Consequently, an exothermic reaction speed is slow or the reaction ceases sometimes during the use.